1. Field of the Invention
The present invention relates to the field of computer networking, specifically to the field of asynchronous transfer mode (ATM) communications. More specifically, the present invention relates to methods and apparatus for providing various network services to nodes in an ATM network by multiplexing service-related messages on a virtual path in the network.
2. Description of the Related Art
The present invention relates to the field of ATM and similar networking systems. Such systems are characterized by use of high-speed switches which act to switch message cells of a fixed size and format through the network. Below is provided a general description of ATM networks
ATM Networks
The preferred embodiment of the present invention is implemented in an asynchronous transfer mode (ATM) network. Such networks are well-known in the an and, in fact, are described in various references. One such reference is Handel, R. and Huber, M. N., Integrated Broadband Networks, an Introduction to ATM-based Networks, published by Addison-Wesley Publishing Company, 1991 and hereinafter referred to as the Handel et al. reference. Another such reference is de Prycher, M., Asynchronous Transfer Mode solution for broadband ISDN, published by Ellis Horwood Limited, West Sussex, England, 1991.
Importantly, in ATM networks information is communicated in fixed-size cells which comprise a well-defined and size-limited header area and user information area. The switch fabric of an ATM switch is designed to act upon information in the header area in order to provide for routing of cells in the network. The switch fabric is normally implemented in hardware, for example using large-scale integrated circuits, in order to provide for high-speed switching of cells in the network.
Standards have been adopted for ATM networks, for example, by the International Telegraph and Telephone Consultative Committee (CCITT). The CCITT standards require a header area comprising a lured set of fields and being of a fixed size and a payload area, also referred to as a user information area, and also of a fixed size but allowing user-defined information fields. The CCITT standards define the header to be of a very limited size to keep at a minimum overhead associated with each cell.
ATM Cell Format
In an ATM network, all information to be transferred is packed into fixed-size slots which are commonly referred to as cells. Of course, such fixed-size slots may be referred to with other terminology, for example, packets. In one standard ATM format (CCITT Recommendation I.150, "B-ISDN ATM Functional Characteristics", Geneva, 1991), the format is generally shown in FIG. 1(a) and includes a 5-byte (also called octet) header field 101 and a 48-byte information field 102. The information field is defined by the standard to be available to the user and the header field is defined by the standard to carry information pertaining to ATM functionality, in particular, information for identification of the cells by means of a label. See, Handel et al., at pages 14-17.
The standardized format for the header field 101 is better shown in FIG. 1(b) and 1(c) and is described in greater detail with reference to Handel et at., at pages 84-91. The header field 101 will be discussed in greater detail below; however, it is worthwhile mentioning here that the header field 101 comprises two fields: (1) a virtual channel identifier (VCI) and (2) a virtual path identifier (VPI). The VPI field is defined as an eight-bit field in one format (see FIG. 1(b)) and as a twelve-bit field in another format (see FIG. 1(c)) and is defined to be used for routing of the cell. The VCI field is also used for routing in the defined format and is defined as a sixteen-bit field.
The de Prycher reference further describes the format of the ATM cell, for example at pages 55-124 and, especially at pages 106-108.
ATM Switching
Two primary tasks are accomplished by an ATM switch: (1) translation of VPI/VCI information and (2) transport of cells from the input port to the correct output port. The functions of an ATM switch are more fully described in Handel et al. at pages 113-136.
A switch is typically constructed of a plurality of switching elements which act together to transport a cell from the input of the switch to the correct output. Various types of switching elements are well-known such as matrix switching elements, central memory switching elements, bus-type switching elements, and ring-type switching elements. Each of these are discussed in greater detail in the Handel et al. reference and each carries out the above-mentioned two primary tasks.
Translation of the VPI/VCI information is important because in a standard ATM network the contents of these fields only has local meaning (i.e., the same data would be interpreted differently by each switch). Thus, the VPI/VCI information is translated by each switch and changed prior to the cell being output from the switch. This translation is accomplished through use of translation tables which are loaded into the switch fabric, generally under control of a switch controller.
ATM Service Providers
An ATM network typically includes at least one service provider whose function it is to provide various services to devices in the network. These services may include, for example, support of a meta-signalling channel specified by the CCITT standard (channel having a VPI:VCI value of 0:1) and supervisory and configuration management functions for internal operations of the devices and for reporting of status of the devices to the service provider.
The various devices (e.g., switches and clients) are interconnected in the network through external ports on the various switches. In the network, these external ports are the lowest addressable unit addressable by the service provider. Importantly, each of the various devices in the network must communicate with the service provider during operation of the network. The communication between the devices and the service provider may be viewed as system overhead and, although necessary, such overhead should be minimized. One area in which an overhead penalty may be paid is in set-up and tear down of communications links between the various devices and the service providers. Typically, it is required to set up at least one communication link per active port. Another area of significant resource overhead is the allocation of VPI/VCI translation table entries and bandwidth utilization on the physical links. For example, each time a communication link is set-up or torn down, translation tables in each switch involved in the communication link must be allocated and updated. Updating of the translation tables and other types of allocation overhead, of course, requires system resources which could be better spent on providing user communication in the network.
Objects of the Invention
Therefore, what is desired is to develop an improved ATM network or the like in which a supervisor or other service provider provides services to various devices in the network.
It is further desired to develop an ATM network in which services are multiplexed on a service path.
It is further desired to develop an ATM network in which service channels are established using predetermined service channel numbers for identification of the service channel.
It is still further desired to develop an ATM network which may be configured in a logical star topology.
It is still further desired to develop a system in which the particular switch and port number of service request can be identified by the service provider at the time the service provider receives a request. In the preferred system, this identification is provided by using the fields in the cell header of the ATM cell and, particularly, by using the VPI/VCI field; no part of the payload area of the cell is utilized for identification information. This leads to efficiency in communication in the network of the present invention.
These and other objects of the present invention will be better understood with reference to the below Detailed Description and the accompanying figures.